Electrophysiological studies of both the isolated renal nephron and the frog skin are being done to assess the interrelationships of the active transport pathways and their passive shunt pathways in these epithelial tissues. In particular for the frog skin, the effects of drugs and hormones on active sodium transport are being investigated as viewed from its current-voltage relationship. This approach allows direct estimates to be obtained of the emf of the sodium pump as well as the values of the electrical resistances of both the active and passive pathways. Studies of the isolated cortical collecting tubules are also being done to assess the nature and contribution of the shunt pathway to the transport characteristics of this nephron segment under conditions of altered Na-K balance of the animals under study. These studies include microelectrode impalements of single cells as well as determinations of the transepithelial characteristics of the tissue studied with the isolated tubule perfusion technique.